Oscillators that have a frequency determining crystal, are commonly used to provide a stable output frequency. However, the crystals used in such oscillators are temperature sensitive and therefore temperature compensating methods are normally required to maintain a stable oscillator output frequency. One accepted method is to generate a temperature varying voltage and apply it across a varactor to control the resonant frequency of the crystal oscillator.
Analog compensation methods have discrete compensation segments each of which work over a wide temperature range. As customers demands for improved frequency versus temperature stability has increased, the degree of matching between the compensation circuit and the crystal has become critical. Hence, precise matching must be maintained over a wide temperature range. This can be difficult to accomplish in a cost effective manner, especially since analog compensation segments can have some interaction between segments.
In connection with digital compensation methods, these methods have had the advantage that each compensation point is better defined for a particular temperature, which makes for improved processing.
Digital temperature compensated crystal oscillators (TCXOs) utilize periodic temperature monitoring techniques, to determine the operating temperature of the crystal at any particular instant in time. Electronic circuitry generates frequency correction signals which compensate a particular crystal's resonant frequency versus temperature characteristics over a given temperature range. Data which is needed by this circuitry to produce the correction signals for a given crystal can be stored into non-volatile memory at the time of manufacture of the oscillator. When used in their end applications, these TCXOs monitor crystal temperature at periodic intervals, retrieve the compensation data from memory for the temperature range which includes the present temperature, and convert that data to the proper correction signal required to tune the oscillator back to the correct nominal output frequency.
Since digital TCXOs approximate the crystal's characteristic frequency-temperature curve with a finite number of temperature segments, this type of oscillator can produce an instantaneous frequency change if the crystal temperature changes sufficiently to cause the compensation circuitry to change to a different temperature segment. This sudden frequency change can cause undesirable effects to systems which require stable reference clocks, if the change occurs during a critical time interval measurement window, such as in global positioning systems. In order to improve the integrity of the reference clock during these critical time periods, sudden frequency and phase shifts caused by temperature compensation updates need to be inhibited at these times.
A device or circuit in connection with a TCXO which can inhibit or disable temperature compensation updates for a certain period of time take critical measurements, would be considered an improvement in the art.